alan,
if you cut the blake line then yes you will have to replace it. the only thing that holds the cylinder in place is a circlip on the back side. the bleed nipple will pull through attached to the cylinder.
If the fitting on the brake line to the cylinder is rounded, You may want to replace that line anyway, regardless if you turn it off or cut it off.
joel
1976 MGB with a 1977 engine